Mobile networks are currently evolving from pure circuit switched (CS) networks towards IP based networks, and by that integrate into IP based infrastructures that are also used for the Internet, the World Wide Web and the datacom industry.
With this trend mobile networks follow the evolution steps of the wireline networks, where VoIP (Voice over IP) via DSL access or via WLAN access is existing technology today.
Mobile operators that install IMS (IP Multimedia Subsystem) networks and offer IMS services want to make these services available also to their subscribers when using GERAN/UTRAN access. Hereto, calls from and to this group of subscribers are routed through the IMS network in order for the IMS service engine to execute the call and call related services. This concept is called IMS Centralized Services (ICS). The IMS centralized Services work item in 3GPP targets at using the CS access for access to IMS services, see 3GPP TS 23.292, and is complemented by IMS Service Continuity, see 3GPP TS 23.237.
In parallel to the ICS trend the packet core is evolving (Evolved Packet Core, EPC) as part of the Evolved Packet System (EPS, which is Evolved Packet Core+E-UTRAN), supporting eUTRAN as new radio access network. As part of this discussion, single radio voice call continuity (SRVCC) has been specified and is further evolved in 3GPP SA2 (see 3GPP TS 23.216), enabling inter domain Handover (HO) of an IMS voice call from Packet Switched (PS) domain to Circuit Switched (CS) domain and vice versa. In Rel-9, SRVCC is combined with ICS for an IMS centralized solution supporting also SRVCC from eUTRAN/PS UTRAN to CS UTRAN/GERAN.
The current SRVCC procedures do not allow having an access transfer from circuit switched to packet switched, (LTE (Long Term Evolution) or HSPA (High Speed Packet Access)) also called “return direction for SRVCC”. The only specified solution is from packet switched to circuit switched here especially from GERAN/UTRAN switched and here especially from GERAN/UTRAN to eUTRAN and from GERAN/UTRAN to HSPA.